Ever heard that exercise can replace a bad diet or crunches can create six-pack abs? Exercise myths like these have been around for decades. But this fact remains clear - exercise is one of the best things a person can do to lower his or her risk for many cancers. This includes colon, breast and endometrial cancers.

Further, regular exercise can help people maintain a healthy weight, reduce stress and strengthen their immune system. It also curbs a person’s risk of diabetes and heart disease. MD Anderson senior exercise physiologist Carol Harrison busts the most common exercise myths to help readers get the best cancer-fighting body exercise has to offer.

Myth: Fat burn can be targeted.

Truth: Working out can reduce overall fat, but people can’t control what part of their body burns the most fat.

“Your body breaks down fat and uses it as fuel when you exercise,” Harrison said. “But your body’s not picky. It’ll burn fat from anywhere in your body, not just the part you’re working the most.”

Truth: Lifting weights tones and shapes the body - it doesn’t create the look of a bodybuilder.

“Women have low levels of testosterone, so they don’t naturally build massive muscles,” Harrison said. “There is nothing wrong with a woman pushing up to 200 pounds on a leg press if she can do it.”

Lifting weights can prevent loss of muscle mass, help build bone density and increase the rate at which the body burns calories to maintain a healthy weight. And maintaining a healthy weight can help fight off diseases like cancer.

Myth: Crunches are the best moves for your core.

Truth: “Crunches are one of the least effective core exercises because they don’t get rid of belly fat,” Harrison said. To shed the extra jiggle, Harrison suggests increasing cardio workouts and adding resistance training that targets the entire core.

It’s important to trim excess belly fat because it can increase a person’s chances of getting heart disease and certain cancers. It also raises a person’s risk for metabolic diseases like diabetes.

Myth: Exercise can erase a bad diet.

Truth: “Exercise by no means makes up for a bad diet,” Harrison said.

Diet and nutrition play a larger role than exercise in weight management and cancer prevention. In fact, some foods actually help protect against certain cancers.

Myth: When a person stops strength training, their muscle turns to fat.

Truth: Muscle can’t turn into fat, just as fat can’t transform into muscle.

“Fat and muscle are two different types of tissue,” Harrison said. When a person stops strength training, he or she loses muscle mass and his or her metabolism slows down. A sluggish metabolism means the body is burning fewer calories at rest, which can lead to weight gain.

Truth: “A person can get all the benefits of exercise whether he’s at the gym or at home,” Harrison said. The key is to exercise smarter, not longer. “To get the most out of a workout, strength train before doing aerobic exercises,” Harrison said.

Here’s why: During a workout, the body activates its limited supply of carbohydrates first. This is the best fuel for short-term and intense exercise, like strength training. After the body has depleted its carbohydrate storage, it starts using fat for fuel. And fat is the best fuel for aerobic exercise.

Aim for at least 150 minutes of moderate physical activity, like brisk walking and slow swimming, each week to reduce cancer risks. Or choose more vigorous activities, like running and fast bicycling, for at least 75 minutes each week.

Myth: Stretch before exercising.

Truth: It’s more effective to stretch after exercise when muscles and joints are warm.

“Stretching before has little to no benefit,” Harrison said. Stretching after can improve performance and flexibility, and helps maintain a healthy range of motion in joints.

Stretching also can reduce stress, decrease muscle tension, and improve circulation and posture.

“The more fit you are, the better chance you have to fight off diseases like cancer,” Harrison said. “So, focus on the true dos and don’ts of exercise, and get moving.”